Multilayer systems and their method of production

ABSTRACT

A multilayer system comprises a seed layer (4) of copper deposited on an electrically insulating substrate (2). The seed layer (4) carries a copper pattern (8). An air fired dielectric (10) encapsulates the pattern (8) and the exposed parts of the seed layer (4). The outer skin of the pattern is oxidized as is also the whole of the seed layer other than where it underlies the copper pattern. The oxidized regions of the copper both form a firm bond with the dielectric and inhibit the migration of oxygen into the inner regions of the conductor pattern (8). Also the oxidation of the seed layer renders it non-conductive and therefore obviates the need for its removal. Through holes (12) produced in the dielectric enable electrical connection to the pattern (8).

CROSS-REFERENCES TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 897,503, filed Aug. 18,1986, now abandoned.

This application is a divisional application of U.S. Ser. No. 06/756 818now U.S. Pat. No. 4,657,778 and the contents of the specification filedinconnection with that application are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to multilayer systems.

2. Description of the Prior Art

Multilayer sytems take the form of a substrate which supports aplurality of layers of electrical conductors separated from one anotherby layers of insulation but selectively interconnected to one another bymeans of through connections in the layers of insulation.

In a previously proposed system each layer of conductors is formed on asubstrate and covered with a dielectric layer which then forms thesubstrate for the next conductive layer. Through holes are formed in thedielectric layer to enable interconnections between adjacent conductivelayers. Such through holes may be formed at the time of screen printingif the conductors are also screen printed or instead be formed later forexample by laser drilling.

The conductive layer is formed by screen printing a thick film (as thetechnique is commonly known) of particulate conductive material on thesubstrate. The conductive film is then heated to promote sintering andadhesion of the layer to the substrate. Thereafter the whole is coatedwith a glass forming dielectric and then fired in a furnace at atemperature from the range of 600° C. and to 900° C. to melt thedielectric and to promote adhesion of the dielectric to the substrate.The conductive material may be of copper or copper based materials. Thescreen printing process is, however, generally limited in resolution.Copper and copper based materials, because they are deposited as smallparticles and not as a solid layer, are subject to oxidation throughoutthe layer and therefore require processing in an inert environment forexample of nitrogen to inhibit oxidation. In this case dielectric glassforming materials must also be processed in a nitrogen environment andthese are generally difficult to fabricate.

Furnaces providing a nitrogen environment are complex and expensive torun. Not all dielectrics are suitable for firing in a nitrogenenvironment, and those that are tend to become somewhat porous. Even ifthe insulator is built up layer by layer the desired density is notachieved and the insulator remains porous.

It is an object of the invention to provide an improved multilayersystem.

SUMMARY OF THE INVENTION

According to the invention there is provided a multilayer structurecomprising a substrate having at least one surface composed of anelectrically insulating matrial, a solid pattern of an oxidisableelectrically conductive material bonded to said surface and a coating ofan air firing dielectric both on said pattern and that part of saidsurface not bonded to said pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

A multilayer system and method of producing the system will now bedescribed, by way of example, with reference to the accompanyingdiagrammatic drawings in which:

FIGS. 1 to 6 are sections through the system at different stages ofproduction; and

FIG. 7 is a section through a modified system.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The improved multilayer system and method of making the system to bedescribed is particularly concerned with the production of a lowerconductive layer and insulator in a multilayer (at least two layers)system. The conductive material is selected to be compatible with airfiring glass insulators and to allow very high definition conductivepatterns to be formed. The conductive material is preferably low costcopper.

As shown in FIG. 1 a solid layer of copper 4 is formed on a substrate 2of a ceramic, a glass, or glass-ceramic material or a coating of one oftheir materials on a metal sheet material to a thickness in the range offrom 0.25 to 0.75 μm (micrometres). The formation of the copper layer iseffected by introducing palladium on to the surface of the substrate andthen effecting an electroless deposition of copper.

The copper layer 4 is then coated with an electrical resist 6 to athickness in the range of from 12 to 50 μm, preferably 25 μm. The resistis exposed to a light pattern corresponding to a desired conductorpattern. The resist is then selectively removed with an etching fluid(see FIG. 2) to selectively expose the copper layer 4 underneath.

The exposed copper layer 4 is then electroplated with solid copper to athickness of 7 μm and the residual resist is removed in a manner wellknown in the art. The result as shown in FIG. 3 is that a thickconductor pattern 8 is built up on a thin conductive layer.

A thick film air-firing dielectric (for example 45 μm thick) 10 is thendeposited to cover the copper layers 4 and 8 (see FIG. 3). The entireassembly is then fired in air at a temperature of between 600°-900° C.so as to form a dense glass based electrical insulator.

The dielectric is preferably deposited by a screen printing process. Thedielectric preferably takes the form of particles of glass suspended inethyl cellulose dissolved in butyl carbitol. After screen printing thedielectric is dried to evaporate the butyl carbitol. The ethyl cellulosethen acts as a binder for the glass particles. When the dielectric isfired the ethyl cellulose burns off and the glass melts or fuses to forma dense substantially non-porous layer.

The firing step also has consequences for the underlying copper inasmuchas the copper is oxidised to a depth of about 2 μm (see FIG. 4). Becausethe depth to which the copper is oxidised depends upon such factors asthe firing temperature, the glassiness of the insulator, and the meltingtemperature of the glass, care must be taken to ensure that oxidationtakes place to the desired depth. The result is that the whole of theseed layer 4 not covered by the conductive layer 8 is oxidised to formeda non-conductive copper oxide while the solid conductors of the layer 8which are 7 μm thick are oxidised only to depth of 2 μm and this doesnot appreciably effect the conductive properties of the conductors.

The net result is that the conductors are buried in an air firingdielectric. It will be appreciated that the oxidation of the outer skinof the solid conductors enables the insulator to become intimatelybonded with the conductor. Furthermore the presence of a glass layer onthe oxidised outer skin of the conductor inhibits further diffusion ofoxygen from the insulator into the conductor.

Finally a through hole pattern is established in the dielectric layer 10(see holes 12 in FIG. 6) to provide access to the conductors 8. Thethrough holes 12 can be produced by laser drilling or by etching orabrading techniques.

The through holes 12 can be produced before the assembly is fired byscreen printing methods, for example. Instead laser drilling andabrading or etching methods can be used.

It will be appreciated that the above described multilayer systemprovides a number of advantages.

The conversion of the copper seed layer into copper oxide provides aglass forming material which has a high adhesion with the substrate 2,the coppper conductors 8 and the dielectric 10. The adhesion of thecopper conductors 8 to the substrate is also improved by the firingsince a copper oxide bond is formed.

In effect the function of the oxidised copper seed layer is that duringthe firing step it should become non-conductive and glass-forming whencombined with oxides such as PbO, BaO and SiO₂ present in the insulator.Thus the glass insulator when molten whets to the copper oxide seedlayer and in effect dissolves it. The copper oxide seed layer also whetsto the ceramic, glass or glass-ceramic substrate and complexes with itand so a very strong bond is formed between the insulator and thesubstrate.

In the absence of the copper oxide seed layer between the substrate andinsulator a whetting agent could be included in the glass insulator topromote adhesion directly between the substrate and insulator.

The conversion of the seed layer to an electrical insulator avoids theneed to remove those parts of the seed layer not underlying the copperconductors.

The need for the introduction of ionic aqueous materials is also avoidedat any stage where they could effect the reliability of the system.

In one modification of the described method following the step ofelectroplating with a copper layer 8 and prior to the step of removingthe residual resist, the copper layer 8 is coated with a film 14 (seeFIG. 7) of a metal which will inhibit diffusion of oxygen into thecopper conductor 8. Metals particularly suitable for this purpose arenickel, palladium, gold, silver, chromium, rhodium or any alloy of thesemetals. The step has the effect of reducing the amount of copperconverted into copper oxide advantageously during the air firing step.The diffusion inhibiting film is particularly needed to coat the copperareas where the through holes are formed since otherwise such areaswould be always exposed to the air and therefore subject to oxidation.In FIG. 7 parts similar to those in FIG. 6 are similarly referenced.

In yet a further modification, the oxidation inhibiting layer is coatedwith a thin film of copper to act as an adhesion layer between theoxidation inhibiting layer and the dielectric following the firing ofthe system.

In yet another modification two resist steps and two copper platingsteps are effected prior to the removal of the excess resist and thecovering of the conductors 10 with dielectric. The first resist is athin layer formed by spinning a wet film resist to produce very highdefinition tracks (for example having a width of 10 μm and a thicknessof 4 μm). The second resist is a thick layer deposited using a laminateddry film resist to produce features such as tracks 25 μm thick and 35 μmwide or sites over which the through holes will be formed in theinsulator.

While the seed layer 4 is described as being formed by electrolessdeposition it will be appreciated that it can equally be formed by athin film or a metallorganic method.

Furthermore instead of copper, any other electrically conductivematerial can be used which has the dual properties of, upon oxidation,becoming both non-conductive and of ceramic or glassy form to provide astrong adhesive bond with ceramic, glass or glass-ceramic substrates anddielectric covering layers.

It will be appreciated that the multilayer system can be double-sided.In such systems the substrate is provided with a plurality of throughholes and a conductive layer is formed on both sides of the substratesimultaneously. At the same time conductive material is deposited in thethrough holes to link the two conductive layers.

While in the embodiment described the seed layer is left in tact on thesustrate throughout the process it will of course be appreciated that itcan be removed from areas other than directly under the conductivepattern prior to the dielectric deposition step. In this event thematerial of the seed layer need not be oxidisable.

Also instead of forming the electrically conductive pattern on thesubstrate with the aid of the seed layer it can be formed by othermethods.

In one method the conductive pattern is formed by sputtering orevaporation of the conductor through a mask. Instead the substrate isuniformly coated by sputtering or evaporation and the conductivematerial selectively removed by wet etching of by sputter or plasmaetching to leave the required pattern.

In another method the conductive pattern is formed on the substrate byan electroless plating method guided by a resist using a catalyst suchas palladium. Instead an auto catalysing electroless plating process canbe used.

In yet another method a sheet of conductive material is directly bondedto the substrate at high temperature to form a chemical bond. Anelectrically conductive pattern would then be produced by selectiveremoval of the material with the aid of a resist.

In each case the dimensions of the electrically conductive material bothin depth and width would have to be substantially greater than the depthto which the conductive material whets or complexes with the glassinsulator during the air firing step.

It will be appreciated that the use of a dielectric which is fired inair provides significant advantages over dielectrics fired in an inertenvironment, for example, in a nitrogen environment.

The range of dielectrics which can be readily used is significantlyincreased over that which could be used in a nitrogen environment. Thedielectrics fired in air are generally more dense primarily due to theincreased amount of free oxygen available during the firing step.

When the substrate is a ceramic it is preferably one selected from thegroup consisting of alumina, aluminium nitride and silicon carbide.

When the substrate is an insulated metal, the metal is preferably oneselected from the group consisting of stainless steel, low carbon steeland copper. Such a substrate is shown in FIG. 7 in which a metal sheet2A is coated with an electrically insulating layer 2B.

The term "multilayer" as used in the present specification is intendedto emcompass two layers or more.

Many modifications can be made to the invention without departing fromthe spirit and scope of the invention as defined by the appended claims.

I claim:
 1. An electrical device comprising a multilayer structure, saidmultilayer structure comprising:a substrate having at least one surfacecomposed of an electrically insulating material; a solid pattern ofelectrically conductive material bonded to said surface, the surfaceonly of the pattern being superficially oxidised; and a coating of anair firing glassy insulator fused both on said superficially oxidisedsurface of the pattern and on that part of said substrate surface notbonded to said pattern.
 2. A device according to claim 1 wherein saidelectrically conductive material comprises copper.
 3. A device accordingto claim 1 wherein said pattern is bonded to said surface by a seedlayer of the same material as said pattern.
 4. A device according toclaim 1 wherein said substrate comprises a metal sheet coated with anelectrically insulating material.
 5. A device according to claim 1wherein said substrate comprises a ceramic.
 6. A device according toclaim 1 wherein said substrate comprises a glass material.
 7. A deviceaccording to claim 1 including a layer of palladium between thesubstrate and the solid pattern of oxidisable electrically conductivematerial.
 8. A device according to claim 1 wherein the solid pattern ofoxidisable electrically conductive material comprises two layers of saidmaterial of different thickness, the thinner layer being located betweenthe thicker layer and the substrate.
 9. A structure according to claim 1wherein the solid pattern of oxidisable electrically conductive materialincludes an outer oxidised layer having a thickness of the order of 2μm.
 10. A device according to claim 1 including a diffusion inhibitorlocated between the solid pattern of oxidisable electrically conductivematerial and the coating.
 11. A device according to claim 10 wherein theinfusion inhibitor is selected from the group consisting of nickel,palladium, gold, silver, chromium, rhodium or an alloy of any of thesematerials.